Use of suspensions of gas microbubbles in a carrier liquid as efficient ultrasound reflectors is well known in the art. The development of microbubble suspensions as echopharmaceuticals for enhancement of ultrasound imaging followed early observations that rapid intravenous injections can cause solubilized gases to come out of solution forming bubbles. Due to their substantial difference in acoustic impedance relative to blood, these intravascular gas bubbles are found to be excellent reflectors of ultrasound. Injecting into the blood-stream of living organisms suspensions of gas microbubbles in a carrier liquid strongly reinforces ultrasonic echography imaging, thus enhancing the visualisation of internal organs. Since imaging of organs and deep seated tissue can be crucial in establishing medical diagnosis a lot of effort is devoted to the development of stable suspensions of highly concentrated gas microbubbles which at the same time would be simple to prepare and administer, would contain a minimum of inactive species, would be capable of long storage and simple distribution. Many attempts towards a solution which will satisfy these criteria have been made, however, none have provided a completely satisfactory result.
It has been known from EP-A-0 077 752 (Schering) that suspensions of gas microbubbles can be made by mixing an aqueous solution of a surfactant with a solution of a viscosity enhancer as a stabilizer. The gas bubbles are introduced into the mixture by forcing the mixture of reagents and air through a small aperture. A suspension of CO.sub.2 microbubbles may be obtained by addition of an acid to a mixture obtained from a solution containing a surfactant and sodium bicarbonate and a solution of the viscosity enhancer. Mixing the components however, is to be carried out just before use and the solution is to be consumed/injected immediately upon preparation. The disclosed surfactants (tensides) comprise lecithins; esters and ethers of fatty acids and fatty alcohols with polyoxyethylene and polyoxyethylated polyols like sorbitol, glycols and glycerol, cholesterol; and polyoxy-ethylene-polyoxypropylene polymers. Disclosed concentration of tensides in the suspension is between 0.01% and 10% wt and a preferred range is claimed to be between 0.5% to 5%. The viscosity enhancing and stabilizing compounds include for instance mono- and polysaccharides (glucose, lactose, sucrose, dextran, sorbitol); polyols, e.g. glycerol, polyglycols; and polypeptides like proteins, gelatin, oxypolygelatin, plasma protein and the like. The total amount of viscosity enhancing agent is limited to 0.5 and 50%. Use of polyoxypropylene-polyoxyethylene polymers (eg. Pluronic.RTM. F-68) as viscosity enhancing agent has also been disclosed. In the preferred example, equivalent volumes of tenside, a 0.5% by weight aqueous solution of Pluronic.RTM. F-68 (a polyoxypropylene-polyoxyethylene copolymer), and the viscosity enhancer (a 10% lactose solution) are vigorously shaken together under sterile conditions to provide a suspension of microbubbles. The suspension obtained lasted over 2 minutes and contained close to 50% of bubbles with a size below 50 .mu.m. According to the document up to 50% of surfactants and/or viscosity enhancing agents may be employed, however, specific examples use between 1% and 4% of Pluronic.RTM. F-68.
Easy-to-produce aqueous suspensions usable as imaging agents in ultrasonic echography are disclosed in WO-91/15244 (Schneider et. al.). The suspensions contain film forming surfactants in laminar and/or lamellar form and, optionally, hydrophilic stabilizers. The laminarized surfactants can be in the form of liposomes i.e. microscopic vesicles, generally spherically shaped. These vesicles are usually formed of one or more concentrically arranged bi-molecular layers of amphipathic compounds i.e. compounds with a hydrophilic and a hydrophobic moieties. The molecules in the bilayers are organised so that the hydrophobic moieties are in facing relationship, the hydrophilic moieties pointing toward the water phase. The suspensions are obtained by exposing the laminarized surfactants to air or a gas prior to or after admixing with an aqueous phase. Conversion of film forming surfactants into lamellar form is carried out according to various liposome forming techniques including high pressure homogenisation or sonication under acoustic or ultrasonic frequencies. The concentration of phospholipids claimed is between 0.01% and 20% and the concentration of microbubbles is between 10.sup.8 and 10.sup.9 bubbles/ml. The microbubble suspensions remained stable for months. The concentration of phospholipids in Example 1 is 0.5%.
An attempt toward a stable echogenic suspension is disclosed in WO-92/11873 (Beller et. al.). Aqueous preparations designed to absorb and stabilise microbubbles for use as an echographic contrasting agent are made with polyoxyethylene/polyoxypropylene polymers and negatively charged phospholipids such as phosphatidylglycerol, phosphatidylinositol, phosphatidylethanolamine, phosphatidylserine as well as their lysoforms. The concentration range of phospholipids in the preparations may be between 0.01% and 5% by volume or weight, however, preparations with 1% of dipalmitoylphosphatidyl glycerol (DPPG) are specifically disclosed and claimed. In addition to the negatively charged phospholipids the compositions must contain between 0.1% and 10% of polymeric material (Pluronic.RTM.F-68). The total amount of solutes in the preparations is between 5.1% and 10.4%. The concentration of the microbubbles is not reported, however, according to the results given it may be estimated to be about 10.sup.7 bubbles/ml. The stability of the suspensions is reported to be better than that of EP-A-0 077 752.
Although the prior art compositions have merit, they still suffer several drawbacks which hamper their practical use. Firstly, some prior art compositions have relatively short life spans and secondly, they have a relatively low initial bubble count e.g. between 10.sup.4 and 10.sup.5 bubbles/ml. This makes reproducibility and analysis of echographic tests made with such compositions fairly difficult. In addition, some techniques produce bubbles in a wide range of diameters (up to 50 .mu.m) which prevents their use as echographic agents in certain applications (e.g. echography of the left heart).
The need for stable formulations of microbubbles which will resist pressure variations in the blood streams and have a good shelf life is further amplified by poor stability of some of the state-of-the-art compositions. Microbubble formulations whose distribution and storage would not present problems are particularly important.
Another drawback is that many of the heretofore known compositions contain a high amount of different solutes such as polymers, phospholipids, electrolytes, and other which render their practical use more and more difficult. For example, it is known that use of polyoxyethylene/polyoxypropylene polymers (Pluronic.RTM.) with particular patients may cause unpleasant side effects (see for instance G. M. Vercellotti et. al. Blood (1982) 59, 1299). Preparations with a high phospholipid content in certain cases may also be undesirable. In any event, compositions with a high degree of various solutes are administered reluctantly and their wide spread use is becoming considered to be undesirable. In fact, the trend in the pharmaceutical industry is to reduce concentrations of active and inactive ingredients in various medical or pharmaceutical formulations to their lowest possible levels and eliminate from the preparations everything that is not necessary. Finding alternative methods and formulating more effective compositions continues to be important. This is particularly so with microbubble suspensions used in echography since here the ingredients have no curative effect and should lead to the least possible after consequences. However, as stated above, the state of the art preparations with typical concentrations in the range of 1% and 4% by weight and the teachings of prior art discourage use of reduced amounts of phospholipids and other non-phospholipid additives. The reason for the discouragement is most probably hidden in the fact that in the course of the routine experimentation further reduction in concentration of the ingredients never produced suspensions which were stable enough to have any practical use or encourage further tinkering in the lower end of the known range.